This application is related to the copending application entitled "Corrosion Resistant Magnetic Film" filed by the same inventors based on Japanese Patent Application No. 276783/87 corresponding to U.S. Pat. application Ser. No. 264,694. The present invention relates to a Magneto-Optical disk in which information is written on magnetic thin films by optical modulation and the information is read out by means of optical modulation using a Magneto-Optical effect such as the magneto-optic Kerr effect, or the like.
Magneto-Optical disks have been used as recording media for optical high-density bulk erasable memories in various fields including as external memories for computers, optical disk files, etc.
In a Magneto-Optical disk a magnetic thin film having perpendicular magnetic anisotropy (that is a Magneto-Optical recording film which hereinafter referred to as a perpendicular magnetic anisotropy film) is formed on a transparent substrate by a thin-film forming technique such as a sputtering, a vacuum evaporation, or the like. Various studies have been done on the film structure of such Magneto-Optical disks.
In those studies, particularly as a method for forming perpendicular magnetic anisotropy films, polycrystalline thin films or the like have been variously examined. Such a material has however disadvantages in that production of material is difficult, large recording energy is required, noise due to crystal grain boundary cannot be disregarded, and so on. Therefore, thin films of amorphous alloys of rare-earth and transition metals, such as TbFe, TbFe, Co, GdTbFeCo, or the like, are the subject of material examination. These thin films are of amorphous alloys of rare-earth and transition metals. However, the rare-earth metals are very easily oxidized. Even if a film structure has a predetermined composition ratio at first, the oxidation of the rare-earth metal progresses as time passes so that the composition becomes rich in the transition metal. In the extreme case, the perpendicular magnetic anisotropy necessary to a high-density memory is lost, or even if the perpendicular magnetic anisotropy is not lost a change in coercive force Hc occurs.
Recently, such a medium structure has been therefore proposed as shown in FIG. 6, in which a foundation protecting layer 2 is formed between a transparent substrate 1 and a perpendicular magnetic anisotropy film 3. A protecting film 4 is formed on the perpendicular magnetic anisotropy film 3 so that the perpendicular magnetic anisotropy film 3 is interposed between the foundation protecting layer 2 and the protecting film 4 to thereby protect the perpendicular magnetic anisotropy film 3 from the front and rear surfaces thereof. A protecting resin layer 5 made of ultraviolet ray setting resin transparent acrylic resin, epoxy resin, or the like is further formed on the protecting film 4.
In the above conventional medium structure shown in FIG. 6, however, the protection of the perpendicular magnetic anisotropy film 3 by means of the protecting films 2 and 4 is not sufficient to prevent the perpendicular magnetic anisotropy film 3 from penetrating moisture and oxygen through the protecting films 2 and 4, so that the magnetic anisotropy film 3 is oxidized or corroded at its interfaces, so as to produce pin holes or generate of selective oxidation of the rare-earth metal only. As a result, there have been problems in that the characteristics of the disk media deteriorate so that the initial characteristics cannot be maintained owing the passage of time.